Published: 2020-02-26 04:31:16 BdST
The German champions, sharper and livelier than their hosts, piled on the pressure from the start of the match and the stats showed they enjoyed 63% possession.
It was a bad night for Chelsea, whose home form under Frank Lampard has been suspect all season, and their misery was complete in the 83rd minute when wing back Marcos Alonso was sent off for violent conduct after he caught Lewandowski in the face with his arm.
For former Arsenal forward Gnabry they were the fifth and sixth goals in London in the Champions League this season after he scored four in Bayern's 7-2 demolition of Tottenham Hotspur in the group stages.
Chelsea, run ragged most of the evening, had barely got their breath back when Lewandowski and Gnabry exchanged passes halfway down the pitch before Gnabry raced away to score.
Lewandowsi, clearly enjoying himself, got among the scorers in the 76th minute after a stirring run from Canadian full back Alphonso Davies.
The Germans had also wasted a series of chances in the first half.
Thomas Mueller, one of three players who figured in Chelsea's 2012 Champions League final victory over Bayern in Munich, went closest in the 35th minute when he thumped the bar with a header from another Lewandowski cross.
Tuesday's win proved a measure of revenge for that unlikely Chelsea victory and the Germans will feel fully confident of progressing when they welcome the London side for the second leg on March 18.